Unbonded flexible pipes are frequently used as flexible risers or flexible flowlines for transport of fluid hydrocarbons such as oil and gas. The unbonded flexible pipes convey the fluids from a hydrocarbon reservoir located under the sea bed to a floating structure. The fluid may be a hydrocarbon fluid, such as natural gas or oil, depending upon the nature of the hydrocarbon reservoir, or an injection fluid such as water. The fluids, which are transported to the floating structure, can be processed, for example by compression and/or further treatment.
Flexible unbonded pipes of the present type are for example described in the standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008. Such pipes usually comprise an inner sealing sheath—often referred to as an internal pressure sheath, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or usually a plurality of armoring layers. Often the pipe further comprises an outer sheath or outer protection layer which provides mechanical protection of the armor layers. The outer protection layer may be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing layers are arranged between armor layers.
In general flexible pipes are expected to have a lifetime of 20 years in operation.
The term “unbonded” means in this context that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the inner sealing sheath and optionally an armor structure located inside the inner sealing sheath normally referred to as a carcass.
Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in a gaseous form because it occupies a smaller volume and does not need to be stored at a high pressure.
The Floating Liquefaction, Storage and Off-loading (FLSO) concept combines the natural gas liquefaction process, storage tanks, loading systems and other infrastructure into a single floating unit. The Floating Liquefied Natural Gas (FLNG) concept is similar to that of the FLSO concept, but additionally provides natural gas treatment as well as the liquefaction process, storage tanks, loading systems and other infrastructure into a single floating structure. Such concepts are advantageous because they provide off-shore alternatives to on-shore liquefaction plants. These vessels can be moored off the coast, or close to or at a gas field, in waters deep enough to allow loading of the LNG product onto a carrier vessel. They also represent movable assets, which can be relocated to a new site when the gas field is nearing the end of its productive life, or when required by economic, environmental or political conditions. When the floating structure is moored close to a gas field or other hydrocarbon reservoir, it can be kept in fluid communication with the producing well heads via one or more flexible risers. The one or more flexible risers are designed to convey fluids between the well heads of a hydrocarbon reservoir and the floating structure. Flexible risers may be configured as free-hanging catenaries or provided in alternative configurations, such as steep and lazy S and wave configurations, using buoyancy modules and tethered buoys. Thus, a flexible riser may be connected at one end to the floating structure, and at another end to a riser base manifold, which secures the flexible riser to the sea bed.
A subsea pipeline connects the riser base manifold to the well heads either directly, or via a well manifold. The subsea pipeline may be a metal or composite tubular flowline, or a flexible flowline comprising flexible pipe. In such configurations, a production hydrocarbon, e.g. natural gas, from a hydrocarbon reservoir, e.g. a gas field, passes along the subsea pipeline from one or more well-heads, which may be in the same or different hydrocarbon reservoirs, to the riser base manifold.
The riser base manifold is the point at which the production and any injection pipelines are connected to one or more flexible risers which convey the production hydrocarbon to the floating structure. The riser base manifold provides the touchdown point at which the flexible riser reaches the sea bed, and the riser base manifold may comprise an end fitting. Alternatively, the flexible riser reaches the sea bed at a touchdown point distant from the riser base manifold to which it is connected. The flexible risers may be connected to the floating structure at a hang-off point. The hang-off point may be at a side of the floating structure, or situated within a moonpool in the floating structure, for example at the bottom of a turret. The floating structure may be moored to the sea bed by a plurality of mooring lines which are anchored to the sea bed.
The flexible pipes used as flexible risers and flexible flowlines are frequently unbonded flexible pipes which are constructed of a number of independent layers, such as helically laid steel and polymeric layers formed around a central bore for transporting fluids. A typical unbonded flexible pipe comprises from the inside and outwards an inner armoring layer known as the carcass, an inner impermeable sheath surrounded by one or more armoring layers and an outer impermeable sheath. Thus, the inner impermeable sheath forms a bore in which the fluid to be transported is flowing. Moreover, the inner and the outer sheath form an annular volume, known as the annulus, which comprises one or more layers of armoring layers and an annular void. However, although the sheaths forming the annular volume in principle are impermeable, during time gases may pass through the sheaths into the annular volume. From the bore of the pipe gasses, such as CO2 and H2S, may permeate through the sheath into the annular volume and cause corrosion of the armoring layers in the annular volume, which are normally made from steel.
The paper titled “Prevention and monitoring of fatigue-corrosion of flexible risers' steel reinforcements” by Antoine Felix-Henry, OMAE2007-29186, published as part of the Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, Jun. 10-15, 2007, San Diego, Calif., USA, discloses a flexible riser pipe comprising steel reinforcing layers contained in an annular volume located between inner and outer polymer sheathes. The structure of the flexible riser is designed to seal and avoid any direct contact between the fluid carried in the bore and the steel reinforcing layers in the annular volume.
Although the inner and outer sheaths defining the annular volume are designed to be leak proof under high temperature and pressure conditions, small amounts of gases can permeate through the inner sheath from the riser fluid. Corrosive gases such as carbon dioxide, dihydrogen sulphide and water vapour may be present in a hydrocarbon production fluid in the bore. Such corrosive gases may diffuse into the annular volume and attack the steel reinforcing layers. OMAE2007-29186 discloses that a gas venting system can be integrated inside the flexible pipe end terminations to flush corrosive gases from the annular volume.
The annular volume may also accidentally be exposed to water, such as from seawater ingress from a damaged outer sheath, or water vapour diffusing from the bore fluids through the inner sheath and condensing in the annular volume, may result in the corrosion of the steel reinforcing layers. Such corrosion is normally undesired as it may result in a reduction in the fatigue life of the riser.
In addition, an annular volume exposed to water ingress may also prevent gas from being vented from the annulus, leading to a pressure build up which may result in a rupture in the external sheath and further corrosion issues.
In response to such ingress of water as a result of damage during use, OMAE2007-29186 discloses that subsea clamps may be installed in the flexible riser to seal the outer sheath at any damaged area and provide a gas release valve from the annular volume. The gas release valve can be applied to a “hog bend” region of the flexible riser. The “hog bend” represents a local maximum in the height of a sinusoidal- or wave-shaped flexible riser. The annular volume can then be injected with an inert fluid to mitigate against corrosion and extend the fatigue life of the flexible riser.
Such a method may result in the leakage of the inert fluid from the annular volume into the surrounding environment. If the inert fluid is not environmentally benign, this method of corrosion mitigation may be associated with a mild degree of environmental damage.
WO-9840657-A1 discloses a riser wherein the annulus is continuously flushed with a lower pressure medium, such as warmed gas or oil.
WO-00/17479 discloses a riser wherein the annulus is connected to the inner channel of the riser via a flow path to prevent over-pressure of the annulus. The flow path includes a one-way valve or a pump to allow fluid to flow from the annulus to the inner fluid channel, but not the other way around. The riser may include a further flow path to introduce fluids or gases in the annulus for cleaning and maintenance thereof.
FR-2 858 841 discloses a method for operating a riser. The method comprises injecting an entrainment gas under pressure in the annulus to force permeated gases in the annulus to flow along towards a vent, to be vented to the outside of the riser.
Under normal circumstances corrosion can be handled in flexible unbonded pipes, and an evenly distributed corrosion in the pipe is normally considered to be acceptable. However, if local conditions concentrate the corrosion to a localized zone, even a modest increased corrosion may have significant impact on local pipe properties and lead to damage.